The in vitro proliferation and differentiation of lymphocytes are regulated by many factors. The most common regulators mainly are soluble cytokines, various natural cells, and genetically engineered recombinant cells. In vitro cell culture, regulation mediated by the genetically engineered recombinant cells expressing a certain cytokines as feeder cells or antigen-presenting cells (APCs) on lymphocytes is often stronger than that by free cytokines, indicating advantage of immobilized cytokines. Therefore, the genetically engineered recombinant cells expressing certain cytokines are commonly used as the feeder cells or APCs to promote the proliferation and activation of lymphocytes. Both the feeder cells and the APCs are auxiliary cells; therefore it is necessary to deactivate their proliferative activity in order to not affect the in vitro proliferation and activation of lymphocytes. The commonly used methods include gamma ray irradiation, UV irradiation, physical methods such as electric shock and chemical treatments such as mitomycin. But there are still many defects, such as inability of large-scale preparation, expensive equipment, unsafe operation, being hard to control the degree of inactivation, possible residual live cells, or utter destruction of cells, etc. In addition, both physical and chemical methods can lead to crosslinking or destruction of nucleic acids. Although the cross-linked or destructed nucleic acids no longer function in cell proliferation, they still remain in the cells as genetic materials and moreover these nucleic acids or even the debris of them may be integrated into other genes, which is a major obstacle for clinical application of the prepared cells.
Therefore, how to obtain an effective and safe in vitro booster of lymphocytes remains an outstanding challenge in biology.